1. Field of the Invention
The present invention relates to a multimode optical fiber and an optical fiber ribbon for optical interconnection and an optical interconnection system employing the optical fiber and the optical fiber ribbon.
2. Description of the Related Art
Methods used for transmitting a signal in a device include an electric transmission system and an optical interconnection system. The optical interconnection system is currently attracting attention because it can achieve a signal transmission with an even broader bandwidth that the electric transmission system, a space saving feature, and a low power consumption. There are two systems for implementing the optical interconnection system: one using an optical waveguide circuit; and the other using an optical fiber. However, because it is preferable that all the optical components used in the device should be housed in a space saving manner, the optical fiber occupies a place as one of the optical components suitable for the optical interconnection, with which a flexible wiring can be made and a signal transmission with low loss is possible.
It is often the case that a multimode optical fiber (MMF) is employed as an optical fiber for a short-haul optical transmission. The MMF usually has a core diameter several times to dozens of times larger than a core diameter of a single-mode optical fiber (SMF), with a larger numerical aperture. Therefore, the MMF offers an easy connection with other optical components, and particularly when connecting with an optical component such as a light source, a connection can be made with low loss without performing a highly precise alignment as the case of the SMF.
A graded-index optical fiber (hereinafter, “GI optical fiber”), which is a sort of the MMF, is an MMF in which an influence of the mode dispersion is suppressed by optimizing the refractive index profile of the core region, which is frequently used for a short-haul optical communication.
In particular, there are a larger number of research reports on an optical transmission system in which an LED having an oscillation wavelength of 850 nm or a vertical cavity surface emitting laser (VCSEL) is used as a light source and a GI optical fiber is used as an optical transmission medium.
The GI optical fiber used in a typical short-haul optical communication is the GI optical fiber having the core diameter of 50 μm and the maximum relative refractive index difference of 1.0% defined in the ITU-T (International Telecommunication Union Telecommunication Standardization Sector) G.651. However, it is difficult to use this GI optical fiber as an optical fiber for the optical interconnection system because it produces a large bending loss when bending the optical fiber for housing it in a device.
In the meanwhile, a method of reducing the bending loss is disclosed in Japanese Patent Application Laid-open No. 2006-47719, with an MMF having an α-powered refractive index profile in which a depressed region having a refractive index lower than that of the cladding is formed on the outside of the core.
However, providing the α-powered refractive index profile to the core and forming the depressed region in the cladding in the optical fiber make the refractive index profile complicated, causing troublesomeness in its fabrication. In addition, because it is required to use a dopant for lowering the refractive index, such as fluorine, there is a problem of increasing the manufacturing cost.